System and method for data collection using near-field magnetic induction

ABSTRACT

A method of collecting location data from objects moving within a specified site includes providing at least one tag reader and a plurality of tags each configured to communicate via near-field magnetic induction across an open-air gap of at least two meters. The tag reader listens for a tag signal transmitted by any one or more of the tags. Each tag transmits a tag signal containing a unique identifier that is received at a tag reader. The tag signal is demodulated to determine the unique identifier associated with the particular tag. Also disclosed is a system for monitoring a condition within a shielded environment using a tag and a tag reader that communicate using near-field magnetic induction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to data collection systems and more particularlyto a system and method for collecting data using near-field magneticinduction.

2. Description of the Prior Art

Data collection and information collected about the movement andbehavior of multiple attendees at a given location has become useful forvarious reasons. For example, data may be collected about the attendeesto a convention or trade show in order to record who attends the eventin general and the particular exhibits visited by the attendees. Dataabout the locations visited by the attendee can be used to determine anattendee's interest level in various displays, attractions, or games ofchance at the event or venue.

Historically, attendee tracking has been handled in many different waysincluding sign-up sheets, name tags with bar codes that can be scanned,NFC-enabled devices such as cell phones or tags, and RFID communicationsystems using RFID tags. Based on the locations visited within thevenue, the data can be used to predict future behaviors of the visitors,such as the level of attendee interest in a certain product, generatingsales leads, or determining demographics and marketing strategy.

One popular method of collecting data about attendees at a venueinvolves the use of either barcodes, or more recently, QR codes. Thebarcode or QR code is printed on the attendee's name badge. When anattendee is interested, for example, in a product being displayed at atrade show, an activity at a theme park, or in a particular game ofchance at a casino, a worker at the venue scans the code on theattendee's name badge and the collected data is stored for analysis orfollow up.

Another method of collecting attendee data is by using transceivers thatcommunicate with each other. For example, a first transceiver remainsstationary at a specific table or booth in the venue. A secondtransceiver is carried by a person moving throughout the venue. As thesecond transceiver approaches the first transceiver, the firsttransceiver sends a query to the second transceiver, which in turn sendsa reply containing its assigned code to the first transceiver. For thepurpose of tracking attendees at an event, these systems can be used toidentify the attendee at a door, station, or area of the event.

One type of tracking system utilizes radio waves to communicate betweentransceiver pairs. Current technologies for this type of system operatein several frequency bands including the 915 MHz, and 2.45 GHz bands.These types of systems are generally referred to as far field becausethe transmissions radiate into free space, where the far field regionbegins approximately 1/10 of a wavelength from the antenna and extendsoutward. Examples of such devices include, but are not limited to, cellphones, RFID tags, and tablet computers.

In contrast to far-field communication systems, some near-fieldcommunication systems use transceiver pairs that operate between 120KHz-135 KHz. These types of systems are generally referred to as nearfield because transmissions are contained within a localized,non-propagating magnetic field or communication “bubble” surrounding acommunication device such as a radio, headset, or microphone. Thisbubble generally extends less than a few feet from the antenna.

In near-field communication (NFC) systems, magnetic field energyemanates from the communication system, but does not radiate into freespace as is the case in far-field transmission. Typically, near-fieldcommunication is at 13.56 MHz and has a range of no more than a fewinches. In this type of system, communication occurs when one device isbrought to within an inch or two of another device. For example, when anNFC-enabled device is touched to a reader, the device is activated bythe reader and transmits a reply signal to the reader. This reply signalcontains the attendee's information, which may be stored in a databasefor follow-up communication.

US patent application publication no. 20120223819A1 to Burgess disclosesa NFC system used to track attendees at a trade show or other event. Toovercome problems with interference, the NFC system of Burgess has arange of less than 10 centimeters. Attendees at the event have an NFCtag built into an ID card or badge. As the attendee enters and leaves abooth or station at the event, the system has a reader that communicateswith the tag as the user passes the tag close to or touches it to thereader. When the attendee places the tag against the reader, forexample, the system records the time in, the time out, the tag holder'sidentity, and the identity of the station visited. Data collection canbe passive or active as long as the user places the tag within the shortrange of the reader.

Another method of attendee tracking involves the use of RFID tags andreaders. Each attendee is issued an RFID tag that is uniquely coded tothe individual. When the attendee approaches a display or game ofchance, an RFID reader reads the attendee's RFID tag and stores theinformation. This method operates at ranges on the order of tens offeet, unlike the NFC-based systems that generally have a range of lessthan 10 cm. Compared to NFC systems, the increased range of RFID systemdoes not require the attendee to manipulate or swipe the tag, making ita hands-free system.

One such RFID system is disclosed in US patent application publicationno. 20080312946 to Valentine for a method of trade show data managementusing RFID and wireless networks such as Bluetooth®. The network is ableto track a visitor's location and movement in the trade show venue bycommunication between the tag and the reader at periodic intervals.Inferences are drawn from the collected data about the attendee'sinterest in certain booths based on proximity to other attendees,interactions with other attendees, multiple visits to a certainlocation, and the time spent at the booth.

SUMMARY OF THE INVENTION

The above-described conventional tag/reader systems have severaldeficiencies. For example, devices are range limited to severalcentimeters (often 4 cm or less), which requires the person carrying thetag to place the tag in very close proximity to the reader. This alsogenerally requires that the person carrying the tag actively participatein the data collection method by choosing to swipe or wave the tag closeto the reader. Alternately, the person carrying the tag must happen toposition himself close enough to the reader for the tag and reader tocommunicate. If the person carrying the tag chooses not to participate,is too lazy to swipe the tag, or just forgets to do so, data is notcollected. Also, if the person carrying the tag fails to swipe the tagboth at entry and upon exit, no duration information is collected forthe person visiting the booth. Incomplete data is not usable in manycases.

NFC-based systems that operate at 13.56 MHz suffer from the deficiencyof requiring the attendee to take action to express interest in atradeshow booth or gaming table. The attendee must locate the NFC readerand then move the NFC-enabled device within an inch or two of thereader. Should the attendee be engrossed in the activity and forget tohave the NFC device scanned, is unable to find the NFC reader, or simplywishes not to participate in the process, valuable data is lost thatwould allow follow-up with that attendee at a later date. NFC systemsrequire short read ranges because they are susceptible to interferencefrom metals, liquids, and other signals. NFC systems also do not havethe ability to read multiple devices simultaneously. Significant amountsof valuable marketing data can be lost due to the limited read range andsingle tag reading deficiencies of this attendee tracking technology.

A deficiency of ultra-high-frequency systems (e.g., 900 MHz and above)is that they are subject to various degrees of signal interference. Thisinterference can include interference from surrounding structures,external radio frequency interference, and interference due to weathersuch as rain and snow. Interference can prevent the transceiver pairsfrom effectively communicating with each other and again result inuncollected data or bad data. Therefore, these systems require a clearline of sight to be effective.

Due to interference problems, RFID systems require significant set-upthat includes mounting an antenna, cabling the antenna back to thereader, and cabling the reader to a storage device that holds captureddata. Additionally, RFID tags must be aligned properly and face thereader antenna in order to be read. Any metal in the area will block thesignals or create multipath situations where received data will becorrupted. The use of unrestricted and unlimited numbers of readers isnot possible due to reader-to-reader interference. This type of systemrequires significant forethought and coordination with all exhibitors orgaming areas to ensure no overlap of reader signals. These deficienciestypically result in significant amounts of uncollected data, advancedplanning to avoid interference issues, and lengthy set-up times.Accordingly, the benefit often does not justify the cost.

It is not easy to create a system in which multiple, arbitrarilypositioned, and arbitrarily oriented attendee tags move relative to afixed tag reader. Some systems have overcome this issue by requiring aclose coupling of the tag reader and the tags in a controlled geometricmanner or by limiting the read point to one tag at a time. Theserestrictions limit the number of applications where automatic datacollection can be applied and also require manipulative action by theuser.

Another deficiency of some prior-art systems is that the tag readercannot simultaneously transmit and receive signals. In many prior-artantennas, the transmit coil and the receive coil are the same coil. Insuch a system with only one coil, a transmit/receive switch switchesbetween the transmitter and the receiver depending on the state of thetransceiver. Therefore, simultaneous transmitting and receiving is notpossible.

Prior-art systems fail to reduce coupling between collocated transmitand receive antennas while also maximizing signal range. In contrast tothese prior-art systems, antennas of the present invention eliminate thecoupling of the transmit signal into the receiver when the transmit coiland the receive coil are collocated on parallel planes or on a singleplane. For example, by placing one or more receive coils inside theclosed geometry of the transmit coil, placing one or more receive coilsoutside of the closed geometry of the transmit coil, and connecting allof the receive coils in series, the transmit signal is eliminated orsufficiently reduced from the receiver. Since the transmit signalvoltages received by the inner and outer receive coils are 180 degreesout of phase, the sum of these signals in the inner and outer receivecoils cancel each other. The sum of the areas enclosed by the turns ofthe inner receive coil (also known as the turns-area product) must equalthe sum of the areas enclosed by the outer coils in order to achievecomplete phase cancellation of transmit signals in the inner and outerreceive coils. If these turns-area products are not equal, phasecancellation reduces, rather than fully eliminates, the local orcollocated transmit signal to the receiver.

For the same reason, when an external signal impinges on the inner andouter receive coils of antennas of the present invention, the signals ofthe inner and outer receive coils are in phase. As a result, the signalsimpinging on the inner and outer receive coils add, thereby increasingthe receiver sensitivity of the system and allowing for an increasedrange of near-field magnetic communication. This antenna structure alsoobviates the need for a transmit/receive switch and all of itscomplexity. Additionally, one could transmit and receive continuously atthe same time using the same frequency or using different frequencies.

Unlike the attendee tracking methods and systems of the prior art, thepresent invention provides an improved attendee tracking or datacollection system that includes attendee tags and readers that operatewith a low-frequency (100 KHz-300 KHz) non-propagating magnetic field.System operation at low frequency allows the system to workvolumetrically (i.e. a tag enters a volume of space where a reader isinterrogating) instead of line-of-sight and around metals and liquids.

Additionally, embodiments of the data collection system of the presentinvention provide a reader with an antenna that allows signaltransmission and reception concurrently in the three spatial planes aswell as transmitting and receiving simultaneously at the same frequencyor at different frequencies. This antenna feature allows the attendeetag to be oriented in any direction relative to the tag reader and stillcommunicate effectively with the tag reader.

Further, tag readers of the present invention can be adjusted to operateat distances from a few inches up to 15 feet using, but not limited to,knobs, buttons, or software. This allows fine tuning of the area beingmonitored for attendee tags by a tag reader and allows for better datacollection as to the exact location or display of particular interest tothe attendee.

Still further, embodiments of the present invention with passive tagsrequire only the tag reader to be powered for the system to operate.Such a tag reader needs no further set-up or cabling. In systems of thepresent invention, the setup typically only involves plugging the readerinto a standard electrical outlet and then setting it down in a desiredlocation, such as beneath a display counter.

Still further, the improved data collection system of the presentinvention uses error correction codes, timing delays, and other means toallow the reader to see multiple attendee tags in its volumetricmagnetic field. The fields from multiple tag readers can also overlapwith no impact on the system's ability to operate.

Still further, in situations where multiple, overlapping tag readerscommunicate with a single attendee tag, the reader that is closest tothe attendee tag can be determined using a received power measurementthat is easily performed in the tag reader, thereby allowing betterparsing of the data to determine which location the attendee wasactually viewing or visiting.

Still further, attendee tags of the system can be passive or activedepending on the desired maximum range. Passive tags can be used forshorter distances of up to a few feet while active, battery powered,tags can be used for longer ranges of up to 15 feet or more.

Finally, the improved data collection system allows for data to bestored on a local reader storage device such as a flash card, or harddisk drive, or be transmitted to a central data base server using any ofmany standard networking protocols either wirelessly or wired.

One aspect of the present invention is directed to a system forcollecting data about objects moving within a specified site. The systemincludes at least one tag reader disposed within a specified site andconfigured to transmit a magnetic field to an interrogation area withinthe specified site and extending at least two meters across an open-airgap. Each tag reader has a transceiver antenna comprising a firstantenna coil disposed on a planar substrate and substantially defining afirst coil closed geometry with an innermost first coil turn and anoutermost first coil turn. At least one second antenna coil is disposedon the planar substrate within the innermost first coil turn. Eachsecond antenna coil has a plurality of second coil turns eachsubstantially defining a closed second coil geometry, where each secondantenna coil defines an effective second coil area. At least one thirdantenna coil is disposed on the planar substrate outside of theoutermost first coil turn and having a plurality of third coil turnssubstantially defining a closed third coil geometry, where each thirdantenna coil has an effective third antenna coil area. The third antennacoil(s) is (are) connected in series with the second antenna coil(s),where the effective third antenna coil area differs from the effectivesecond coil area by no more than 5%, thereby resulting in at least 95%phase cancellation between a second voltage induced in the at least onesecond antenna coil and a third voltage induced in the at least onethird antenna coil when conducting a current through the first antennacoil. The system also includes a plurality of tags configured towirelessly communicate with the tag reader(s) via near-field magneticinduction across an open air gap of at least 2 meters. Each of the tagsis constructed to be carried by an object moving within the specifiedsite. When located within an interrogation area, each of the tags isconfigured to wirelessly communicate a unique identifier to therespective tag reader via near-field magnetic induction.

In one embodiment, the first antenna coil is a transmitter antenna. Thesecond antenna coil(s) and the third antenna coil(s) comprise a receiverantenna.

In another embodiment, the first antenna coil is a receiver antenna. Thesecond antenna coil(s) and the third antenna coil(s) comprise atransmitter antenna.

In some embodiments, the effective third antenna coil area differs fromthe effective second coil area by no more than 1%, thereby resulting inat least 99% phase cancellation between the second voltage induced inthe at least one second antenna coil and the third voltage induced inthe at least one third antenna coil when conducting the current throughthe first antenna coil.

In another embodiment, the transceiver antenna is configured as afocused-beam directional antenna constructed and arranged to direct themagnetic field within a particular angular region to define theinterrogation area.

In another embodiment, the magnetic induction has a frequency from 100KHz to 300 KHz.

In another embodiment, each tag reader is configured to transmit andreceive at the same time.

Another aspect of the invention is directed to a method of collectinglocation data from objects moving within a specified site. In oneembodiment, the method includes the steps of providing at least one tagreader with a tag reader antenna configured to receive signals vianear-field magnetic induction across an open-air gap of at least twometers; providing a plurality of tags configured to wirelesslycommunicate with the tag readers via near-field magnetic inductionacross an open air gap of at least 2 meters; each tag reader listeningfor the tag signal transmitted by any one or more tags; each of theplurality of tags transmitting a tag signal containing a uniqueidentifier; receiving a tag signal from a particular tag at the at leastone tag reader; demodulating the tag signal from the particular tag todetermine the unique identifier associated with the particular tag; andsaving the unique identifier.

In one embodiment, the providing step includes selecting the tag readerwith the tag reader antenna configured as a transceiver antenna capableof transmitting and receiving at the same time.

In another embodiment, the tag signal comprises a plurality of tagsignals separated in time by a constant tag signal interval. In oneembodiment, the tag signals are separated in time by a varying tagsignal interval. In some embodiments, the tag signal interval isadjustable or programmable by a user.

In some embodiments, the magnetic induction has a frequency from 100 KHzto 300 KHz.

In another embodiment, the method also includes the steps of selectingthe at least one tag reader to transmit a magnetic field to aninterrogation area extending across an open-air gap of at least twometers; the at least one tag reader transmitting the magnetic field tothe interrogation area; and at least one of the plurality of tagsreceiving the magnetic field.

In another embodiment, the tag reader antenna is configured as adirectional antenna and the interrogation area is substantiallyconstrained to a specific angular region.

In another embodiment, the method includes equipping at least one objectwith one of the plurality of tags.

In another embodiment, the tag periodically transmits a signal to (e.g.,“pings”) a passive receiver. In this embodiment, the reader does notinterrogate, but instead listens for signals transmitted by one or moretags across an open air gap of at least 2 meters. For example, eachattendee tag transmits a tag signal at predefined intervals, where thetag signal contains a unique identifier. Each tag reader listens for tagsignals transmitted by any of the attendee tags. After receiving a tagsignal from a particular attendee tag, the signal is demodulated todetermine the unique identifier associated with the particular attendeetag. The unique identifier may then be saved by the system.

Another aspect of the present invention is directed to a method ofmonitoring an enclosed environment. The method includes the steps ofdisposing a sensor inside an environment enclosed by an enclosure madeof metal; disposing a transmitter inside the environment, where thetransmitter configured to communicate using near-field magneticinduction; coupling the sensor to the transmitter; disposing a receiveroutside the enclosure, where the receiver configured to communicate withthe transmitter using near-field magnetic induction; coupling thereceiver to a communications device outside of the enclosed environment;wirelessly communicating a sensed condition of the enclosed environmentfrom the transmitter to the receiver using near-field magneticinduction; and communicating the sensed condition to an user using thecommunications device.

In one embodiment, the communications device is selected as a computer,a display panel, a wireless network, or a cabled network.

In one embodiment, the enclosure is selected as a refrigerator, afreezer, an oven, a liquid-storage tank, a cargo trailer, or a cargocontainer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing components of a system for collectingattendee data at an event.

FIG. 2 is a block diagram of an embodiment of a system of the presentinvention.

FIG. 3 is a block diagram of a magnetic flux antenna component of thesystem of FIG. 1.

FIG. 4 is a block diagram of a tag reader component of the system ofFIG. 1.

FIG. 5 is a block diagram of an attendee tag of the system of FIG. 1.

FIG. 6 is a block diagram of an embodiment of an adjustable gainamplifier component as included in the embodiment of the tag reader ofFIG. 4 and the attendee tag of FIG. 5.

FIG. 7 is a circuit diagram of a voltage controlled resistor componentof one embodiment of the adjustable gain amplifier of FIG. 6.

FIG. 8 is a plan view of one embodiment of a transceiver of the presentinvention showing a printed circuit board that includes an inner receivecoil, a plurality of outer receive coils, and a transmit coil positionedradially between the inner receive coil and the outer receive coils.

FIG. 9 is perspective diagram of the transceiver of FIG. 8 substantiallytaken along line A-A and illustrating the theory of operation of thetransceiver.

FIG. 9A is a sectional view of part of the transceiver of FIG. 8 showingan outer receive coil canted with respect to the transmit and innerreceive coils.

FIG. 9B is a sectional view of part of the transceiver of FIG. 8 showingan outer receive coil in a plane parallel to the transmit coil, wherethe effective area of the outer receive coil is reduced due to the angleof incidence of the magnetic field.

FIG. 10 is a plan view of another embodiment of a transceiver of thepresent invention showing a printed circuit board that includes anoctagonal transmit antenna on one side of the circuit board and fourreceive antennas on the opposite side of the circuit board.

FIG. 11 is a perspective diagram of another embodiment of a transceiverof the present invention showing a transmit coil and receive coilspositioned inside and outside of the transmit coil.

FIG. 12 is a plan view of another embodiment of a transceiver of thepresent invention showing a transmit coil and a receive coil, where thearea of the receive coil that is inside the transmit coil issubstantially equal to the area of the receive coil that is outside thetransmit coil, thereby providing phase cancellation.

FIG. 13 is a plan view of another embodiment of a transceiver of thepresent invention showing an inner receive coil, a transmit coil, and aplurality of outer receive coils, where the inner and outer receivecoils are connected in series.

FIG. 14 is a plan view of another embodiment of a transceiver of thepresent invention showing a plurality of receive coils connected inseries, an inner transmit coil, and an outer transmit coil, where thetransmit coils are also connected in series and the receive coils aredisposed radially between the inner transmit coil and the outer transmitcoil.

FIG. 15 is a flow chart illustrating steps of one embodiment of a methodof collecting data of attendees at an event.

FIG. 16 is a flow chart illustrating additional steps of the method inFIG. 15.

FIG. 17 illustrates one embodiment of a system for monitoring anenvironment within a shielding enclosure.

DETAILED DESCRIPTION

Embodiments of the present invention are illustrated in FIGS. 1-17.

Referring to FIG. 1, one embodiment of the current invention is directedto a system 100 for collecting data about objects 2 a moving within aspecified site. Object 2 a may be a person, animal, package, suitcase,or other item of interest. For example, objects 2 a are attendees movingabout an event held at the specified site or venue. The specified sitecan be a room, building, town, or other area.

System 100 includes a plurality of uniquely-coded tags 2 (also referredto as attendee tags 2 in some embodiments) assigned and distributed toeach object 2 a. For example, when system is configured for attendees atan event, each object 2 a (i.e., attendee) is assigned an attendee tag 2during event registration, where the attendee tag 2 is coded with aunique identifier. Optionally, the attendee tag 2 contains theattendee's name, company name, contact information, and the like. Inanother example, when system 100 is used to track animals or inanimateobjects 2 a, tag 2 may contain the identity/name, owner's name,inventory number, or other relevant information.

System 100 also has one or more tag readers 1 placed at a location 30within the specified site, such as an event exhibit, a tradeshow boothwithin an exhibit hall. Each tag reader 1 is positioned to collect dataabout objects 2 a. For example, system 100 collects the uniqueidentifier of each attendee tag 2 that visited booth 30 or any of booths31, 32, 33, the length of visit for each attendee tag 2, and the numberof return visits (if any) by attendee tag 2. System 100 may also be usedto monitor visits to a specific location 30 such as, for example, whenobject 2 a is a person visiting a particular display 33 a at booth 33 ofa trade show. The unique identifier of each attendee tag 2 maycorrespond to the identity of the object 2 a holding the attendee tag 2.This information can be used to develop sales leads and marketingstrategy. In other embodiments where system 100 configured for trackinganimals or inanimate objects, the collected information may be used, forexample, to confirm the location of each object 2 along a journey,assembly line, delivery route, process, or the like.

In one embodiment, tag reader 1 has a tag reader antenna 5 with threetransceivers set-up orthogonally to each other in an X-Y-Z orientation.This arrangement of tag reader antenna 5 is a magnetic flux antenna thatcan receive tag signals 150 from any direction. Optionally, where thedirection of the magnetic field 130 of tag reader 1 needs to be directedto a specific space or region, tag reader 1 may have paramagnetic orferromagnetic materials used to direct the shape or direction ofmagnetic field 130 as is discussed in more detail below.

As also discussed in more detail below, one embodiment of system 100 hastag reader antennas 5 with transmit and receive coils located in thesame or parallel planes. This antenna design permits tag reader antenna5 to transmit and receive at the same time, which is unlike traditionaltransceiver antennas that alternate between receive and transmit. Byconfiguring the tag reader antenna 5 to transmit and receive at the sametime, in addition to modulating interrogation field 46, system 100 isconfigured to continuously interrogate. As a result, system 100 can seemultiple tags 2 at one time in interrogation field 46 because thevarious tags 2 each periscope at different times based on when they werestarted and the set length of time between each periscope. Unliketraditional transceivers, tag reader 1 does not need to shut down theinterrogation field 46 (transmit) to listen for responses from the tags2 (receive).

In one embodiment, tag reader 1 modulates the interrogation field 46 sothat tags 2 distinguish the interrogation field 46 from random noise.Each tag 2 recognizes the modulated signal and determines that the tag 2can communicate with tag reader 1, as opposed to receiving random noise.For example, the tag reader 1 has a transmitter circuit and a receivercircuit that operate somewhere between 100 KHz and 300 KHz. Tag reader 1uses active filtering in the receiver to reject out-of-band signals thatcould corrupt the response signal of tag 2. In some embodiments, tagreader 1 also uses bit error correction algorithms to recover corruptedtag information whenever possible. When tag 2 detects a readerinterrogation field 46, tag 2 will respond by transmitting reply data totag reader 1.

In one embodiment, tag 2 has a directional tag antenna 15 to increaseits range. In other embodiments, tag antenna 15 is omnidirectional. Forexample, the tag antenna 15 is a coil wrapped around a piece ofhigh-permeability ferrite or other high-permeability material. Tag 2 maybe either passive or active. For example, a passive tag 2 withininterrogation field 46 has a capacitor that is charged by interrogationfield 46. In response to interrogation field 46, tag 2 transmits its IDnumber to tag reader 1.

When tag 2 is an active tag, for example, it periscopes at apredetermined interval. In one embodiment, the interval is from thirtyseconds to sixty seconds; in other embodiments, the interval is one totwo seconds. Interval length may be chosen depending on the desireddetail of information collected. At each periscope, tag 2 listens for asignal from interrogation field 46. If tag 2 does not detect a readerinterrogation field, it will go back to sleep and periscope at the nextappointed interval. If tag 2 detects or “hears” a signal in a readerinterrogation field 46, it will respond by transmitting its unique ID,go back to sleep, and then periscope again at the next appointedinterval. In one embodiment, periscoping in active tag 2 is accomplishedusing a processor watchdog timer that helps preserve the battery life.In some embodiments, the time between periscopes or transmissions fromeach tag 2 is spaced by a fixed, variable, or random length of time. Insome embodiments, the length of time is adjustable and/or programmableby the user.

In some embodiments, the active tag 2 has a tone detector circuit usedto determine if it is in a reader interrogation field 46. Based ondetecting an identifiable tone sequence while in reader interrogationfield 46, the tone detector circuit will wake up tag 2 and then transmitthe unique tag ID.

In other embodiments, tags 2 are active and tag reader 1 is passive,where active tags 2 transmit a signal periodically (i.e., ping) to tagreader(s) 1. In such an embodiment, tag reader 1 listens for pings fromthe active tags 2 that are within range and records the tag informationafter receiving a ping.

FIG. 2 illustrates an embodiment of system 100 that includes a tagreader 1 and one or more tags 2. Each tag 2 has a tag antenna 15. Eachtag reader 1 has at least one tag reader antenna 5. As discussed in moredetail below, one embodiment of tag reader antenna 5 includes a firstreader antenna oriented in an X plane, a second reader antenna orientedin a Y plane, and a third reader antenna oriented in a Z plane. In oneembodiment discussed further with reference to FIG. 8, each transceiverof tag reader antenna 5 has an inner receive coil 510, one or moretransmit coils 520, and one or more outer receive coils 530, where thetransmit and receive coils 510, 520, 530 are coplanar or on parallelplanes and where signals received by the inner and outer receive coils510, 530 from the transmit coil 520 substantially cancel each other dueto phase cancellation.

Optionally, each tag 2 is configured to be moved with respect to tagreader 1 at a variety of different velocities represented by velocityvectors 4. In some embodiments of system 100, the direction in whichtags 2 approach tag reader 1 is significant. For example, a trade showexhibit 30 (shown in FIG. 1) that backs up to an aisle is close enoughto objects 2 a (shown in FIG. 1) walking in the aisle such that the tags2 of these objects 2 a communicates undesirably with a tag reader 1 inthat booth 30. As a result, objects 2 a merely walking in the aislebehind the booth 30, but who do not visit or see its contents, could beerroneously recorded as visitors to the booth 30. Thus, by directingmagnetic energy 130 into a specific region 108, tag reader 1 correctlyand effectively communicates with tags 2 at the booth 30 withoutconstraining the distance between tag reader 1 and tags 2. Accordingly,system 100 may be configured to direct magnetic energy 130 into aspecific angular region 108, while also directing that same magneticenergy 130 in a plurality of orientations with respect to multiple tags2 and tag reader 1.

In response to magnetic energy 130 of tag reader 1, tag 2 responds witha reply signal 150. After tag reader 1 demodulates a reply signal 150from tag 2, tag reader 1 transmits a signal 90 to data storage device 3where signal 90 contains a unique identifier corresponding to aparticular tag 2. Data storage device 3 records the unique identifier aswell as location and time data for the tag 2.

Tag reader 1 is optionally coupled to data storage device 3, such as forstorage and management of collected data. In some embodiments, datastorage device 3 is integrated with tag reader 1 into a single unit sothat data is not lost if the communications network fails. In otherembodiments, data storage device 3 is a separate unit coupled to tagreader 1 by wires or wireless communication. Data storage device 3 maybe one of many devices known in the art and include computer harddrives, memory sticks, computers, and the like.

FIG. 3 shows a cross-section of one embodiment of tag reader antenna 5(shown in FIG. 1) that is configured as a directional magnetic fluxantenna 5′. Directional magnetic flux antenna 5′ includeselectromagnetic windings 43, a paramagnetic housing 41, and one or moreferromagnetic elements 42 to direct a magnetic field 130 in a particulardirection over an extended distance. Housing 41 and ferromagneticelements 42 direct a magnetic field 130 across an open-air gap ofinterrogation field 46 between tag reader 1 and tags 2 (shown in FIG.1). Being an open-air gap, interrogation field 46 may be as small as 1cm or as large as 5 meters or more. In one embodiment, interrogationfield 46 is up to 5 meters, and may operate within a specific range,such as a range of 2 to 5 meters.

Directional magnetic flux antenna 5′ both focuses magnetic flux duringtransmit and receives flux about multiple rotational orientations. Whentransmitting, antenna flux lines of magnetic field 130 are focused asshown in FIG. 3 along one side of magnetic flux antenna 5′. This effectoccurs due to the use of paramagnetic and ferromagnetic materials (e.g.housing 41 and elements 42, respectively) arranged about multiplewindings 43. The combination of paramagnetic housing 41, ferromagneticelements 42, and windings 43 creates a magnetic circuit. The magneticcircuit concentrates magnetic flux lines 44 within paramagnetic housing41 and ferromagnetic materials 42, as well as directing a concentrationof flux lines 44 across open-air gap of interrogation field 46 to apoint in space that is external to magnetic flux antenna 5′. The shapeof magnetic field 130 is a function of the placement of paramagnetichousing 41, ferromagnetic elements 42, and electromagnetic windings 43.By adjusting the spacing and orientation of these components, tag readerantenna 5 (shown in FIG. 1) may be configured to function as afocused-beam antenna. Magnetic flux antenna 5′ may optionally omitparamagnetic housing 41 and ferromagnetic elements 42 to provide anomni-directional magnetic flux antenna.

In one embodiment, housing 41 is constructed of mu-metal or equivalentparamagnetic material. Mu-metal is a nickel-iron alloy with highpermeability and effective screening of low-frequency magnetic fields.Housing 41 preferably has an inner sphere 41″ within an outer sphere41′, where spheres 41′, 41″ of housing 41 are separated by a distance ofapproximately 5-15 mm. An opening 49 is located at one pole of housing41. Coil windings 43 are located within the inner sphere 41″.Ferromagnetic elements 42 are placed in inner sphere 41″ to shapemagnetic field 130 and focus the magnetic field 130 to a target.Ferromagnetic elements 42 may be mu-metal or other ferromagneticmaterial. Magnetic flux antenna 5′ may have various other shapes,including flat panels and rectangular boxes with an open side. Tag 2preferably has tag antenna 15 configured as an omni-directional antenna,but tag antenna 15 optionally is configured as a directional magneticflux antenna similar to as described above for tag reader 1.

FIG. 4 illustrates the internal functional blocks of an embodiment oftag reader 1. Tag reader 1 includes tag reader antenna 5, a transmitpower amplifier 6, a receiver amplifier 7, a peak detector 8, one ormore RF tone detectors 9, an analog to digital converter 10, a digitalto analog converter 11, one or more RF burst generators 12 (i.e.,modulators), a tag reader microcontroller 13, a summer 14, and a digitalto analog converter 29. In one embodiment, tag reader antenna 5 is afocused-beam magnetic flux antenna.

Tag reader antenna 5 is electrically coupled with transmit poweramplifier 6 and receiver amplifier 7. Transmit power amplifier 6 isadditionally electrically coupled with D/A converter 29, and summer 14.D/A converter 29 is also electrically coupled with tag readermicrocontroller 13, which receives control signal 129 to adjust the gainof transmit power amplifier 6. Summer 14 is additionally electricallycoupled with the plurality of RF burst generators 12. Each RF burstgenerator is also electrically coupled with tag reader microcontroller13.

Receiver amplifier 7 is also electrically coupled with peak detector 8,D/A converter 11, and each RF tone detector 9. Peak detector 8 isadditionally electrically coupled with A/D converter 10, which iselectrically coupled with tag reader microcontroller 13. Each RF tonedetector 9 is electrically coupled with tag reader microcontroller 13and with receiver amplifier 7. D/A converter 11 is electrically coupledwith tag reader microcontroller 13.

Receiver amplifier 7 has a gain value that is determined by the strengthof tag signal 150 impinging on tag reader antenna 5. Tag reader antenna5 communicates received signal 110 to receiver amplifier 7. Receiveramplifier 7 has multiple gain stages set by a programmable voltage value114 from tag reader microcontroller 13 and converted to an analogvoltage 112 by DA converter 11. Peak detector 8 sends detected peakvalue 118 to A/D converter 10, which converts peak value 118 to adigital peak value 120. Using digital peak value 120 to estimate theamplitude of tag signal 150 impinging on tag reader antenna 5, softwarecalculations of microcontroller 13 determine the value of each stage'sgain. Amplified signal 116 from receiver amplifier 7 is also input to aplurality of RF tone detectors 9. Tone detector 9 demodulate signal 116to provide an RF tone 117 value of 1, 0, or a symbol. For example, atone frequency of 122 KHz is demodulated by tone detector 9 to yield avalue of 1; a tone frequency of 127 KHz is demodulated by tone detector9 to yield a value of 0. Data or identification values are a combinationof the 1s and 0s in RF tones 117 received at tag reader microcontroller13.

Protocol firmware within tag reader microcontroller 13 monitors thepower at each stage and increases or decreases attenuation to maintainthe power level within a prescribed predefined range. The range is basedon the sensitivity of the chosen components of system 100 (shown in FIG.1). By averaging the power levels at each stage with an averagingalgorithm, the software of microcontroller 13 also provides a more evenresponse of system 100. Power monitoring and averaging calculations areperformed in real time as the distance changes between tags 2 and tagreader 1 within interrogation field 46.

Firmware within tag reader microcontroller 13 sends signal 128 thatenables a particular RF burst generator 12 to generate RF burst signals122. RF burst generators 12 are toggled on and off by signal 128 frommicrocontroller 13. Each RF burst signal 122 is input to summer 14 andthe summed RF burst signal 124 is transmitted to power amplifier 6. Inone embodiment, RF burst signal 122 is a sine wave with a frequency thatcorresponds to a value of 1, 0, or a symbol. Transmit power amplifier 6transforms RF signal burst into a drive current 126 that is conductedthrough tag reader antenna 5. Drive current 126 is preferably on theorder of several amperes, but the value of drive current 126 may differdepending on the current that system 100 (shown in FIG. 1) is designedto use. Additionally, D/A converter 29 provides gain control signal 125to transmit power amplifier 6 to adjust the range of tag reader 1. Tagreader 1 transmits an RF signal 130 that impinges on tag antenna 15 oftags 2 (discussed below). RF signal of magnetic field 130 has aparticular modulation scheme and data structure that is unique to tagreader 1. In one embodiment, Amplitude Shift Keying (ASK) is used toprovide a modulation scheme for magnetic field 130 that varies theamplitude. Other modulation schemes can be used that vary the amplitude,frequency, phase, or any combination of these.

FIG. 5 illustrates the internal functional blocks of an embodiment oftag 2. Tag 2 includes a tag antenna 15, a receiver amplifier 16, atransmit power amplifier 17, one or more RF tone detectors 18, an RFburst generator 19 (i.e., modulator), and a tag microcontroller 20. Ineach tag 2, tag antenna 15 is preferably an omnidirectional magneticflux antenna. In some embodiments, tag antenna 15 is a directionalmagnetic flux antenna as described above for tag reader antenna 5 (shownin FIG. 1).

Tag antenna 15 is electrically coupled with receiver amplifier 16 andtransmit power amplifier 17. Receiver amplifier 16 is additionallyelectrically coupled with RF tone detectors 18, which are eachadditionally electrically coupled with tag microcontroller 20. Transmitpower amplifier 17 is disposed in communication with RF burst generator19, which is additionally disposed in communication with tagmicrocontroller 20.

RF signal of magnetic field 130 impinges on tag antenna 15 and a RFsignal 131 is communicated to receiver amplifier 16. Receiver amplifier16 sends amplified signal 133 to each tone detector 18 for demodulation.Each tone detector 18 sends detected tones 135 to tag microcontroller20. Tag 2 reacts by powering on when tag microcontroller 20 detects aparticular RF signal. Tag microcontroller 20 then generates a serialdata stream 132. RF burst generator 19 receives the serial data stream132 from tag microcontroller 20. Serial data stream 132 from tagmicrocontroller 20 toggles RF burst generator 19 on and off. RF signal134 from RF burst generator 19 (e.g., a sine wave) is passed to transmitpower amplifier 17 and has a frequency that corresponds to a value of 1,0 or a symbol. RF signal 134 is converted to a current 136 by transmitpower amplifier 17 and driven through tag antenna 15 to transmit tagsignal 150 to tag reader 1.

Each tag 2 detects a particular frequency of magnetic field 130transmitted by tag reader antenna 5 (shown in FIG. 1). The frequency ofmagnetic field 130 is between 100 KHz and 300 KHz in one embodiment.Upon detection of frequency of magnetic field 130, tag 2 powers up. Tag2 then replies to tag reader 1 by transmitting a tag signal 150 with atone burst of magnetic energy. This tag signal 150 impinges upon tagreader antenna 5.

In one embodiment, tag 2 is a key fob that may be identified using asequence of numbers in burst signal 134. In another embodiment, tag 2 isa smart card, badge, or other portable device that iscommunication-operable with near-field magnetic inductance. In anotherembodiment, tag 2 has only a transmitter section and transmits itsinformation at predefined or random intervals.

FIG. 6 shows one embodiment of receiver amplifier 7 (shown in FIG. 4)configured as an adjustable gain amplifier 50 for received signal 110.Adjustable gain amplifier 50 is described here as it may be used in tagreader 1 (shown in FIG. 1); however, adjustable gain amplifier 50 may beused as amplifier 7 of tag reader 1 and/or receiver amplifier 16 (shownin FIG. 5) of tags 2 and corresponding peak detector, A/D converter, D/Aconverter, and microcontroller. As used in the embodiment of tag reader1 shown in FIG. 4, adjustable gain amplifier 50 has at least one stage(i.e., stage 1, stage 2, . . . , stage n), each stage having one or morevoltage controlled resistors 25 and one or more operational amplifiers26. Adjustable gain amplifier 50 is electrically coupled to tag readermicrocontroller 13, digital to analog converter 11, peak detector 8, andanalog to digital converter 10. Gain is adjusted by the control of thetag reader microcontroller 13 using intelligence embodied in software intag reader microcontroller 13. Gain is adjusted dynamically via thevoltage controlled resistors 25 coupled at each operational amplifier 26(i.e., gain stage). It is contemplated that voltage-controlled resistors25 may be placed at any point in the operational amplifier's feedbackpath. Any of amplifiers 6, 7, 16, 17 (shown in FIGS. 4 and 5) maycomprise multiple gain stages (i.e., stage 1, stage 2, . . . , stage n)as illustrated in FIG. 6.

For example, receiver amplifier 7 of the tag reader 1 shown in FIG. 4 isconfigured as adjustable gain amplifier 50. In this embodiment, tagreader microcontroller 13 sends signal 114 to digital to analog (D/A)converter 11. Based on signal 114 received by D/A converter 11, D/Aconverter sends voltage 112 to each of voltage-controlled resistors 25.Voltage 112 determines the resistance and therefore the gain of eachamplifier stage (i.e., stage 1, stage 2, . . . , stage n). Voltagesettings are retained over time within tag reader microcontroller 13.Output signal 116 from final stage (stage n) of adjustable gainamplifier 50 is sent to peak detector 8, which measures and sends peakamplitude value 118 to analog to digital (A/D) converter 10. Digitalvalue 120 is sent to the tag reader microcontroller 13. The software oftag reader microcontroller 13, with knowledge of the voltage controlledresistor 25 settings and the output peak amplitude value 118, adjustsvoltage controlled resistor 25 setting to maintain the output voltage112 from D/A converter 11 to be within the range of operationalamplifiers 26.

FIG. 7 shows an implementation of voltage controlled resistor 25 thatenables tag reader microcontroller 13 to control a gain stage as shownin FIG. 6. An arrangement of field effect transistors (FETs) enables theresistance between nodes F₁ and F₂ to be set over a wider range thanpossible with a single field effect device. Specifically, the resistancerange is increased by the combination of FET series resistances in eachof columns Col. 1, Col. 2, . . . , Col. N along each of rows Row 1, Row2, . . . , Row K. The adjustment of the resistance value occurs with theuse of cross-column FETs. Tag reader microcontroller 13 software orfirmware calculates the voltage values of V₁₁ through V_(KN), as well aspeak V_(KN). With this arrangement, resistance between node F₁ and F₂ isprecisely adjusted across a wide range of voltage values across nodes F₁and F₂.

FIG. 8 shows a plan view of one embodiment of a transceiver antenna 500of the present invention useful as tag reader antenna 5 (shown in FIG.1). Transceiver antenna 500 includes a transmit antenna 504 and areceive antenna 506. In one embodiment, transceiver antenna 500 has asubstrate 502 upon which transmit antenna 504 and receive antenna 506are disposed. For example, substrate 502 is a circuit board withtransmit antenna 504 and receive antenna 506 disposed as conductivetraces. Receive antenna 506 includes one or more inner receive coil(s)510 and one or more outer receive coil(s) 530. The inner receive coil510 is connected in series with the outer receive coils 530, whichthemselves are connected in series. Thus, all of the receive coils 510,530 are connected in series.

In one embodiment as shown, receive antenna 506 has one inner receivecoil 510 and a plurality of outer receive coils 530. In the embodimentshown, there are sixteen outer receive coils 530. One or more innerreceive coils 510 are positioned within a transmit coil inner areacircumscribed by transmit antenna 504. Here, innermost turn 520 adefines the transmit coil inner area. A plurality of outer receive coils530 are positioned outside of transmit antenna 504. For example,transmit coil 520 is positioned outside of an outermost turn 510 b(e.g., outer diameter) of inner receive coil 510, inner receive coil 510is positioned inside of an innermost turn 520 a (e.g., inner diameter)of transmit coil 520, and outer receive coils 530 are positioned outsideof an outermost turn 520 b (e.g., outer diameter) of transmit coil 520.Inner receive coil 510 may be one or more coils.

In one embodiment, each of coils 510, 520, 530 has a circular or spiralshape. Other shapes are acceptable and coils 510, 520, 530 do not haveto have the same shape. For example, coils 510, 520, 530 may betriangular, rectangular, polygonal, or other regular or irregular shapesprovided that the shape has a closed or substantially closed geometry.

In some embodiments, such as shown in FIG. 8, transmit antenna 504 andreceive antenna 506 are coplanar or substantially coplanar as is thecase when disposed on a single face of a planar substrate 502, such as aprinted circuit board. In use, transmit antenna 504 is connected to atransmitter 270 and receive antenna 506 is connected to a receiver 260configured for receive voltage 261. In other embodiments, transmitantenna 504 and receive antenna 506 are disposed in parallel planes,such as when transmit coil 520 and receive coils 510, 530 are disposedon opposite, parallel faces of substrate 502 or when disposed on otherstructures. For example, coils 510, 520, 530 are made of wire wound oncylindrical substrates 502 and positioned to achieve the samefunctionality as when substrate 502 is a printed circuit board.

Inner receive coil 510 has an inner receive coil axis 511, each outerreceive coil 530 has an outer receive coil axis 531, and transmit coil520 has a transmit coil axis 521, where axes 511, 521, 531 extendperpendicular to a plane of the respective coil (also shown in FIG. 9).In one embodiment, each inner receive coil axis 511, each outer receivecoil axis 531, and transmit coil axis 521 are all parallel to oneanother. In some embodiments, inner receive coil axis 511 and transmitcoil axis 521 are the same axis.

Receive coils 510, 530 and transmit coil 520 each have a closed orsubstantially closed shape, such as a circle, a rectangle, a triangle,or another shape. The term “substantially closed” acknowledges that fora coil with a plurality of concentric turns that spiral between asmaller innermost turn and a larger outermost turn, the adjacent turnsdo not overlap to close the shape. Thus, where one turn of the spiralshape passes the next turn, a substantially closed geometry is defined.Turns of coils 510, 520, 530 may be concentric (e.g., a planar coil),overlapping (e.g., wire wrapped in overlapping turns around a rod), orin a spiral configuration (e.g., wire wrapped in a spiral along a rod.)

FIG. 9 is a simplified sectional diagram illustrating the function oftransceiver antenna 500 of FIG. 8 as viewed along line A-A. As notedabove, an inner receive coil 510 is disposed inside the areacircumscribed by innermost turn 520 a of transmit coil 520 as viewedalong transmit coil axis 521. Transmit coil axis 521 is the same asinner receive coil axis 511, but this does not have to be the case. Aplurality of outer receive coils 530 are disposed outside of outermostturn 520 b of transmit coil 520. Outer receive coils 530 have outerreceive coil axis 531. When transmit coil 520 is active (i.e., poweredby transmitter 270), a transmit loop current 522 conducted throughtransmit coil 520 generates a magnetic field 524 around transmit coil520 according to Ampere's Law.

Illustrated on the right-side of FIG. 9, magnetic field 524 produced bythe transmit coil 520 impinges on outer receive coil 530 in a downwarddirection to induce an outer receive coil voltage 532 in outer receivecoil(s) 530. Magnetic field 524 impinges on inner receive coil 510 in anupward direction to induce an inner receive coil voltage 512 that is˜180° out of phase with outer receive coil voltage 532. It is understoodthat magnetic field 524 would be mirrored on the left side of FIG. 9with similar effect, but is not shown in FIG. 9 for clarity. Accordingto the right hand rule, transmitter magnetic field 524 will pass throughthe inner receive coil(s) 510 (upward) and the outer receive coil(s) 530(downward) in substantially opposite directions relative to each other.Therefore, transmitter magnetic field 524 induces outer receive voltages532 in outer receive coil(s) 530 and inner receive voltages 512 in innerreceive coil(s) 510 that are out of phase with each other. By designingantenna 500 such that inner receive coil(s) 510 and outer receivecoil(s) 530 are connected in series and produce the same magnitude ofvoltage (i.e., ΣV₅₁₂=−ΣV₅₃₂) when excited by the transmit coil 520,receiver 260 (shown in FIG. 8) sees no signal generated by transmitmagnetic field 524 from receive antenna 506 (shown in FIG. 8) duringtransmission because inner receive voltages 512 from inner receivecoil(s) 510 cancel outer receive voltages 532 from outer receive coil(s)530 and vice versa. This phase cancellation results from inner receivecoil(s) 510 being located within the area circumscribed by transmit coil520 and outer receive coil(s) 530 being located outside of transmit coil520.

To protect the circuitry of a receiver 260 (shown in FIG. 8), whentransmit coil 520 generates a magnetic field, the sum of voltagesinduced in series-connected outer receive coils 530 substantially cancelthe sum of voltages induced in inner receive coil(s) 510. In someembodiments, these voltages completely cancel each other or approximatezero when transmit coil 520 generates a magnetic field duringtransmission. Preferably, the phase cancellation results in attenuationby at least 99%. More preferably, phase cancellation results inattenuation of at least 99.4%. Phase cancellation of a smaller degree isalso acceptable depending on the design of antenna 500 and theassociated circuitry of transmitter 270 and receiver 260 (shown in FIG.8).

For example, when the voltage induced in inner receive coil 510 is 100.0v, the voltage induced in outer receive coils 530 is −99.4 v. Therefore,due to series connection of inner and outer receive coils 510, 530 andthe opposite polarity or phase cancellation of the induced voltages,these voltages combine with 99.4% phase cancellation for a resultingreceive signal 261 (shown in FIG. 8) of 0.6 v.

To achieve this result with coplanar transmit and receive antennas 504,506 (shown in FIG. 8), the sum of areas enclosed by turns of the innerreceive coil(s) 510 is equal to or approximately equal to the sum ofareas enclosed by turns of the outer receive coil(s) 530. If, however,the plane of inner receive coil 510 or outer receive coil(s) 530 arecanted at an angle θ (shown in FIG. 9A) with respect to the plane oftransmit coil 520 the effective area of the canted receive coils 510and/or 530 is reduced by multiplying by the cosine of the cant angle θ.Similarly, as shown in FIG. 9B, when one or more of receive coils 510,530 are on a plane parallel to, but axially spaced apart from a planecontaining transmit coil 520, the actual area of receive coils 510, 530is also reduced to the effective area by multiplying by the cosine ofangle θ.

For the purpose of this application, the “effective area” of a cantedcoil with cant angle θ>0 means the area of a hypothetical coil that iscoplanar to the transmit coil 520 and that would produce the same outputvoltage as the canted coil, where the physical area of the hypotheticalcoil is less than the physical area of the canted coil. The effectivearea accounts for the magnetic field lines that may no longer impinge ina direction perpendicular to the receive coils 510, 530. Since the valueof cos(θ) will be equal to 1 or less, the actual area of a coil may needto be similarly increased to compensate for cant angle θ to achieve thedesired result. For 100% voltage cancellation, V_(inner)+V_(outer)=0.While 100% voltage cancellation is ideal, other lesser amounts ofcancellation are acceptable and depend on the particular design needs ofthe transceiver and associated circuitry. For example, phasecancellation of the inner and outer receive coils 510, 530 is used toattenuate the associated receiver voltage 261 (shown in FIG. 8) to be onthe same order of magnitude as, below the noise floor of, or to adesired percentage of receiver voltage 261 (shown in FIG. 8) as measuredby the RMS receiver input voltage of receiver 260 (shown in FIG. 8).Generally, it is desirable to be able to filter out or distinguish thereceiver voltage 261 (shown in FIG. 8) associated with voltage inducedin receiver coils 510, 530 by transmit coil 520.

Expressed in general mathematical terms,

m1Σ₁ ^(n1) A _(inner) cos(θ1)=m2Σ₁ ^(n2) A _(outer) COS(θ2)  (1)

-   -   where    -   m1 is the number of outer receive coils 530 (e.g., m1=16);    -   n1 is the number of turns in an outer receive coil 530 (e.g.,        n1=10);    -   m2 is the number of inner receive coils 510 (e.g., m2=1);    -   n2 is the number of turns in each inner receive coil (e.g.,        n2=20);    -   A is the area enclosed by a single turn of a particular coil        (for a circle, A=πr²)    -   Θ1 is the angle of cant between the inner receive coil plane and        the transmit coil plane; and    -   Θ2 is the angle of cant between the outer receive coil plane and        the transmit coil plane.

In the event that outer receive coils 530 and inner receive coil(s) 510are located in different planes, inner receive coil voltage 512 may beless than 180° out of phase with outer receive coil voltage 532;however, the general principle of phase cancellation still applies toreduce receiver voltage 261 (shown in FIG. 8) to an acceptable level forreceiver 260 (shown in FIG. 8).

On the other hand, as illustrated in the left-hand side of FIG. 9, whenan external magnetic field 526 (i.e., a magnetic field not generated bythe collocated transmit coil 520 of transceiver antenna 500) impinges oninner receive coil 510 and on outer receive coil 530, inner receivevoltages 513 and outer receive voltages 533 induced in the receive coils510, 530, respectively, are in phase since none of receive coils 510,530 is circumscribed by the external antenna coil generating externalmagnetic field 526. That is, no phase cancellation occurs since both theinner receive coils 510 and outer receive coils 530 are located outsideof the external antenna coil (not shown) generating the externalmagnetic field 526. Also, external magnetic field 526 impinges on innerreceive coil 510 and outer receive coil 530 in substantially the samedirection (e.g., downward direction). Therefore, inner receive voltages513 and outer receive voltages 533 are in phase and combine to provide alarger receiver voltage 261 to receiver 260 (shown in FIG. 8). As aresult, the additive combination of in-phase receiver voltages 513, 533effectively increases the sensitivity of the receiver antenna 506 (shownin FIG. 8) so that one may better receive a signal from a desiredexternal magnetic field 526.

As appreciated by one of skill in the art, full cancellation of thecoupling between transmit coil 520 and receive coil(s) 510, 530 becomesmore difficult to achieve as the axial spacing between the coil planesis increased beyond one half of the transmit coil diameter. Also, whenthe lateral spacing between coils exceeds one to two times the diameterof transmit coil 520, it is very difficult to achieve full cancellationin a configuration that does not also cancel all or most of the externalsignal from external magnetic field 526 that is desired to be received.

FIG. 10 shows a plan view of another embodiment of a transceiver antenna500 of the present invention with transmit coil 520 and one or morereceive coils 550. In one embodiment, substrate 502 is a printed circuitboard, where transmit coil 520 and receive coils 550 are disposed aselectrical traces on opposite sides of the printed circuit board. Asshown in FIG. 10, transmit coil 520 has an octagonal shape with aplurality of turns that each define a substantially closed geometry.Transmit coil 520 is disposed on a first side 502 a (back side) ofprinted circuit board 502 and coupled to transmitter 270. One or morereceive coils 550 (e.g., four receive coils 550) are disposed on asecond side 502 b (front side) of the printed circuit board andconnected in series. Each receive coil 550 has a plurality of turns thatdefine a substantially closed geometry. As viewed in the plan view ofFIG. 10, transmit coil 520 intersects each receive coil 550 so that thevoltage induced in each receive coil 550 by the magnetic field fromtransmit coil 520 is partially or completely cancelled as a result ofphase cancellation.

Since part of each receive coil 550 is inside the area enclosed bytransmit coil 520 and part of each receive coil 550 is outside the areaenclosed by transmit coil 520, the transceiver antenna 500 of FIG. 10eliminates the need for distinct inner and outer receive coils. Asdiscussed above with reference to FIG. 9, receive coils 550 in thisembodiment are connected in series to increase the sensitivity oftransceiver antenna 500 to desired signals impinging thereon.Considering the inner-most turn 551 of lower left receive coil 550 a ofreceive coils 550 shown in FIG. 10, inner area 552 inside transmit coil520 substantially equals outer area 553 located outside transmit coil520. Among receive coils 550, inner areas 552 and outer areas 553 aresized so that voltage induced into receive coils 550 during transmissionresults in a zero (or near zero) receiver signal 261 at receiver 260.The phase cancellation during transmission, however, does not cancelvoltage induced in receive coils 550 by an external magnetic field 526generated by a transmitter distinct from transceiver antenna 500 (shownin FIG. 9.) Therefore, transceiver antenna 500 can transmit and receiveat the same time with the same or different frequencies.

To achieve complete phase cancellation (or near-complete phasecancellation) in the embodiment of FIG. 10, the sum of the outer areas553 of receive coils 550 that are located outside turns of transmit coil520 need to be equal to (or substantially equal to) the sum of the innerareas 552 of receive coils 550 that are enclosed within turns oftransmit coil 520. Transmit coil 520 and receive coils 550 in theembodiment of FIG. 10 can be any closed or substantially closed shape,including the square and octagonal shapes as shown.

Referring now to FIG. 11, a plan view illustrates another embodiment oftransceiver antenna 500 shown with inner receive coil 510 with aplurality of receive coil turns, transmit coil 520 with a plurality oftransmit coil turns, and a plurality of outer receive coils 530 each ofwhich has a plurality of outer receive coil turns. Inner receive coil510 is connected in series with outer receive coils 530 and receiver260. Transmit coil 520 is connected to transmitter 270. Each of innerreceive coil 510, transmit coil 520, and outer receive coils 530 has asubstantially closed geometric shape, where inner receive coil 510 islocated within transmit coil 520. As with other embodiments, forsubstantially complete phase cancellation, the sum of the areas forturns of outer receive coils 530 is equal to or substantially equal tothe sum of the areas for turns of inner receive coil 510.

Referring now to FIG. 12, a plan view illustrates another embodiment ofa transceiver antenna 600 coupled to receiver 260 and transmitter 270.Transmit coil 610 has a plurality of turns that each define asubstantially closed geometry. Receive coil 620 has a plurality of turnsthat each define a substantially closed geometry. Transmit coil 610 andreceive coil 620 are rectangular as illustrated, but other closed orsubstantially closed shapes are acceptable. Transmit coil 610 andreceive coil 620 are located on closely-spaced parallel planes, such asopposite faces of a planar substrate 502 (shown in FIG. 10). Receivecoil 620 is positioned to overlap transmit coil 610 to define an innerreceive coil region 622 located within the closed geometry of transmitcoil 610. Receive coil 620 also defines an outer receive coil region 624located outside the closed geometry of transmit coil 610. Inner receivecoil region 622 has an area equal to or substantially equal to the areaof outer receive coil region 624 to the extent required for phasecancellation to reduce receive signal 261 to the desired signal levelfor receiver 260.

In one embodiment, the effective area of inner receive coil region 622is 99%, 99.4% or 99.7% of the effective area of outer receive coilregion 624 (or vice versa). Therefore, during transmission usingtransmit coil 610, a voltage induced into receive coil 620 by transmitcoil 610 results in a zero or near zero signal from transmit coil 610due to phase cancellation. As noted above, due to the phase cancellationof voltage induced by transmit coil 610, receive coil 620 may be used toreceive signals from outside magnetic fields 526 (shown in FIG. 9) atthe same time transmitter 270 is transmitting.

Referring now to FIG. 13, a plan view illustrates another embodiment ofa transceiver antenna 500. Similar to the embodiment of FIG. 8,transceiver antenna 500 of FIG. 13 includes transmit coil 520 with aplurality of turns that define substantially closed geometry. Innerreceive coil 510 also has a plurality of turns each defining asubstantially closed geometry. Inner receive coil 510 is located withinthe closed geometry of transmit coil 520. Located outside of transmitcoil 520 are a plurality of outer receive coils 530 each having aplurality of turns that each define a substantially closed geometry.Outer receive coils 530 are connected in series with inner receive coil510 and receiver 260. Transmit coil 520 is connected to transmitter 270.In one embodiment, transmit coil 520 and receive coils 510, 530 arecoplanar on a substrate 502, such as a printed circuit board.

Referring now to FIG. 14, a plan view shows another embodiment oftransceiver antenna 700 with an inner transmit coil 710, a plurality ofreceive coils 720, and an outer transmit coil 730. Receive coils 720 aredisposed outside of inner transmit coil 710 and within the area enclosedby outer transmit coil 730. Receive coils 720 each have a plurality ofturns defining a substantially closed geometry. Receive coils 720 areconnected in series with receiver 260. Inner transmit coil 710 isconnected in series with outer transmit coil 730 and transmitter 270. Amagnetic field generated by inner transmit coil 710 impinges on receivecoils 720. Similarly, a magnetic field generated by outer transmit coil730 impinges on receive coils 720. Due to the right-hand rule, thevoltage induced in receive coils 720 by inner transmit coil 710 will be180° out of phase with the voltage induced in receive coils 720 by outertransmit coil 730. Similar to embodiments of transceiver antenna 500,600 discussed above, the sum of voltages induced in receive coils 720cancel the voltages induced by inner and outer transmit coils 710, 730due to being out of phase or of reverse polarity with each other.

In use, embodiments of transceiver antennas 500, 600, 700 of the presentinvention include collocated transmit antenna 504 and receive antenna506 that utilize phase cancellation of voltages induced in the receivecoils from a magnetic field generated by collocated transmit antenna504. As a result, the receiver voltage 261 induced by the collocatedtransmitter coil 520 is reduced sufficiently due to phase cancellationto be received by receiver 260 so that it does not damage or overloadreceiver 260, can be filtered as noise from desired external signals, orboth. Accordingly, transceiver antennas 500, 600, 700 of the presentinvention allow for simultaneous transmission and reception using thesame frequency or using different frequencies.

During near-field communication, a transceiver antenna is provided thatincludes collocated transmit coils 520, 610, 710, 730 and receive coils510, 620, 720 each defining a substantially closed geometry. Thetransmit coil circumscribes one or more areas of the receive coil(s).One or more additional areas are positioned outside of the geometry ofthe transmit coil. Thus, during transmit, voltage induced into thecircumscribed areas of the series-connected receive coil(s) isattenuated by voltage induced into the one or more additional areaspositioned outside of the geometry of the transmit coil. In someembodiments of the method, transmission is performed with a firstfrequency and reception is performed using a second frequency, where thefirst frequency and the second frequency may be the same or differentfrequencies. In some embodiments, phase cancellation is used toattenuate by at least 99% a signal induced into the receive coils by thecollocated transmit coil.

With careful design and iterations to fine tune the precise voltagesgenerated by the inner and outer receive coils 510, 530, cancellationapproaching 100% can be achieved. Cancellation in excess of 99.5% canreadily be attained even with standard production tolerances. The amountof cancellation required by a particular system depends on severalsystem parameters, including (1) the relative strength of the signalbeing transmitted by the collocated transmit coil compared to the(desired)) signal being received from a remote system, (2) the limit ofthe absolute maximum input voltage to the receiver which would causedamage to the receiver or excessive electrical non-linearity in thesignal path that would result in reception errors (e.g., receiveroverload), (3) the required signal-to-noise-ratio (SNR) needed toachieve the desired bit-error-rate (BER), where the SNR is generally inthe range of about 6 dB to 15 dB depending on the data coding scheme,(4) if different frequencies are used for transmission and receptionthen the selectivity of any RF filters used to reject the unwanted localtransmitter frequency can compensate for less than 100% cancellation inthe antenna, and (5) less cancellation in the antenna requires filterswith higher selectivity which will add to the cost and size ofequipment. When the transmit frequency and the receive frequency are thesame or nearly the same, it may not be physically or economicallypossible in some cases to design a filter to separate them. Thus, phasecancellation can be used to achieve the desired system performance.

In embodiments discussed above, it is also possible to reverse thetransmit antenna 504 with the receive antenna 506. Since mutual couplingis always reciprocal, either coil set can be used for transmit orreceive. The choice of which coils are used for transmission andreception is determined by the configuration that most easily allows theoptimum number of turns in the respective coils to achieve the bestimpedance match to the transmitter or receiver electronics. Generally,transmitters work best with lower impedance loads (therefore fewer coilturns) and receivers work best with higher impedance coils (requiringmore turns) in order to generate the greatest signal voltage. However,it is not always necessary that the receiver coils 510, 530 have ahigher impedance (greater effective number of turns) than the transmitcoil 520.

Typical frequencies are between 100 kHz and 300 KHz or as limited by thephysical dimensions of antenna 500, 600, 700. In some cases, forexample, receive antenna 506 operates at 133 kHz while transmit antenna504 operates at 215 kHz for communication distances on the order of tensof feet.

Transceiver antennas 500, 600, 700 of the present invention can beimplemented, to collect data from attendees 2 at a venue, such as atrade show, casino, or other venue with attractions. Transceiverantennas 500, 600, 700 may also be used, for example, for securecommunication, access control, and communications for miners. Forexample, transceiver antenna 500 is incorporated into a badge readerthat communicates with an employee badge to allow access to securefacility areas.

Referring now to FIG. 15 and with continued reference to the Figuresdiscussed above, a flow chart illustrates steps of one method 800 ofcollecting attendee data. In step 810, at least one tag reader 1 isprovided, where the tag reader 1 has a tag reader antenna 5. In someembodiments, the tag reader antenna 5 has transmitter coil(s) on thesame plane or on a plane parallel to receiver coils, where a signaltransmitted from the transmitter coils is substantially or completelycanceled by phase cancellation in the receive antenna. In someembodiments, the tag reader 1 has three such coplanar or parallel planeantennas with the three antennas arranged along X, Y, and Z axes. Usingan antenna of this type, the tag reader 1 is configured for continuousinterrogation since the transmit and receive functions may be performedat the same time. This is unlike prior-art antennas that use a switch toalternate between transmission and reception to avoid damage to thereceiver circuitry. In other embodiments, tag reader antenna 5 may justconsist of a receive coil(s).

Optionally, step 810 includes modulating, by a RF tone detector 18, theRF initiator signal 130 in intensity, phase, and/or frequency. In oneembodiment, RF initiator signal 130 is modulated using Amplitude-ShiftKeying modulation. Other forms of modulation are acceptable, such asmodulating phase and/or frequency.

In step 815, the tag reader 1 transmits a magnetic field across the openair of an interrogation area 46 defined by the tag reader antenna 5. Insome embodiments, the magnetic field is modulated for continuousinterrogation. In some embodiments, the magnetic field is transmittedfrom about 100 KHz to about 300 KHz; other frequencies are acceptabledepending on the desired range and power level of the transmitter.

In step 820, a plurality of tags 2 are provided, where each tag 2 has aunique identifier or code. Optionally, tags 2 may each be assigned to aparticular object 2 a. Typically, an tag 2 may be encoded with a uniqueidentifier code, but optionally is configured to transmit the attendee'sname, company name, contact information, and/or other information usefulto the user. Tags 2 in some embodiments have a tag antenna 15 configuredfor increased range and to enable the tag 2 in any orientation toreceive a signal from magnetic field 130 transmitted by tag reader 1. Inone embodiment of method 800, the tag reader antenna 5 is configured asa focused-beam antenna 40. In another embodiment of method 800, the tagreader antenna 5 is configured as an omnidirectional antenna.

In step 825, the tags 2 “listen” for signals as the object 2 a movesabout the venue. In one embodiment, each tag 2 periscopes at predefinedintervals. Tags 2 may have different periscope intervals. In embodimentswhere tags 2 have the same periscope interval, the various tags 2periscope at different times due to being started at different times.Therefore, tag reader 1 is able to communicate with a plurality of tags2. In some embodiments, the tags 2 are configured to identify amodulated RF signal. As such, tags 2 can distinguish between a magneticfield transmitted from a tag reader 1 and other undesirable signals. Inone embodiment, tags 2 demodulate a received RF signal.

In step 830, after receiving the magnetic field 130 while located ininterrogation area 46 of tag reader 1, tag 2 transmits a reply tagsignal 150 to the tag reader 1. In some embodiments, the X, Y, and Zplanes of the tag reader antenna 5 enable the tag reader 1 to receivetag signals 150 from tags 2 that are positioned in any orientation.

In step 835, the tag reader 1 receives the tag signal 150 and stores theinformation in a data storage device 3, such as a database, or passesthe information to another device.

Optionally, in step 840, data storage device 3 or other devicedetermines the identity or other information associated with the tag 2based on comparing the unique identifier value with a stored value. Datastorage device 3 also records locations (e.g., booth number) and timesof day that object 2 a was present at the booth. Depending on theperiscoping interval, the data collected may be, for example, a stringof time stamps spaced by the periscoping interval and a locationassociated with each time stamp as available. When an object 2 a isoutside of range of any tag reader 1, no data is collected from the tag2. The collected information may be analyzed and used as needed formarketing, predicting attendee behavior, identifying possible salesleads. In other variations on the method, the information is used toconfirm the reported location of an attendee or to alert staff to thepresence of an attendee entering an unauthorized area.

Referring to FIG. 16, flow charts illustrate embodiments of steps 815,830, and 835 of method 800. Step 815 includes sub-steps 815 a-815 e,step 830 includes sub-steps 830 a-830 d, and step 835 includes sub-steps835 a-835 b.

In step 815 a, tag reader 1 generates at least one RF burst 122. In step815 b, a summer 14 is used on the at least one RF burst 122 to produce asummed RF burst signal 124. In step 815 c, a power amplifier 6transforms the summed RF burst signal 124 into a drive current 126. Instep 815 d, the drive current 126 is conducted through the windings 43of the tag reader antenna 5. In step 815 e, an RF signal 130 istransmitted across interrogation field 46 via magnetic induction to atleast one tag 2.

In step 830 a, a tag 2 receives the RF signal 130 across the open airgap of interrogation field 46. The RF signal 130 may optionally containa unique identifier comprising a sequence of real, imaginary, or complexnumbers. Each tag 2 has a magnetic flux tag antenna 15 and at least oneRF tone detector 18. Tone detectors 18 are demodulators that determinewhether an incoming signal 133 represents a value of 1, 0 or a symbol.

In step 830 b, each tag 2 detects a frequency of the RF signal 130. Instep 830 c, based on the frequency of the RF signal 130, a particulartag 2 generates a serial data stream 132 having a unique identifiercorresponding to the particular tag 2. Step 830 c of generating a serialdata stream 132 may optionally include generating a plurality of RFbursts 134 in the tag 2 and amplifying the plurality of RF bursts 134 bya transmit power amplifier 17.

In step 830 d, tag 2 transmits tag signal 150 that includes a tone burst134 containing the serial data stream 132.

In step 835 a, the tag reader 1 receives the tag signal 150 at a tagreader antenna 5. In step 835 b, the tag reader 1 demodulates the toneburst 134 in tag signal 150 to provide a unique identifier value.

In one embodiment, method 800 may optionally include the step ofamplifying the receiver tone burst 134 with an adjustable gain amplifier17. Amplification may be done using an adjustable gain amplifier havingone or more stages. The adjustable gain amplifier may utilize a voltagecontrolled resistor 25 in each of the gain stages.

In some embodiments, method 800 is performed with signals from about 100KHz to about 300 KHz. In one embodiment of method 800, the magneticfield 130 and/or the tag signal 150 are between about 120 KHz and about135 KHz.

In use, a tag reader 1 is positioned at a gateway, booth, or otherlocation of interest in a venue. As an object 2 a with the tag 2approaches a booth, for example, tag reader 1 communicates with tag 2via magnetic induction to identify the tag 2 and to record its locationand time of day. Tag 2 may be a key fob, smart card, or other item withRF communication capability that may be identified using a sequence ofnumbers in tag signal 150 sent to tag reader 1.

In an example of system 100, each tag 2 has a unique code such as aserial number. Tags 2 may also store an object identifier code or value.Each tag 2 opens its receiver (i.e., periscopes) for a short time(˜milliseconds) at a specified interval. When the tag 2 receives a queryRF signal 130 from tag reader 1 (e.g., a box located in an area ofinterest), the tag 2 transmits tag signal 150 containing the objectidentifier code and serial number using Amplitude Shift Keying (ASK).This is done one bit at a time or as multi-bit symbols, depending on theparticular implementation. Additional information can be stored in tag 2and transmitted using ASK if requested by tag reader 1.

For example, if using FSK as a modulation scheme, system 100 may utilizefour specific frequencies, one for each symbol. The desired symbol issent from tag microcontroller 20 to RF burst generator 19, which setsthe frequency of transmitted signal 134 to the correct frequency for thesymbol. Tag reader 1 receives magnetic field 150 with frequency 150′ anddemodulates it back to the two bits. All of the data to be transmittedmay be transmitted in this way, which requires significantly less timethan transmitting the data one bit at a time.

As tag 2 is moved closer to the tag reader 1, signal strength increases.Amplifier inputs and outputs are monitored by each component'smicrocontroller 13, 20 and gain is increased or decreased depending onmovement of the tag 2 relative to the tag reader 1. Typical outputvoltage from amplifiers 6, 7, 16, 17 is on the order of singlemillivolts, but depends on the level chosen by microcontrollers 13, 20.

Tag readers 1 and tags 2 use an agreed-upon ASK modulation schemebecause it provides robust digital modulation of magnetic fields 130 andtag signals 150. RF tone detectors 9 of tag reader 1 demodulate incomingsignal 116 and send detected tone 117 to tag reader microcontroller 13.Tag reader microcontroller 13 determines decodes detected tone 117 as a1, 0, or a symbol. Similarly, tone detectors 18 in tag demodulateincoming signal 133 from amplifier 16. In one embodiment, tag readerantenna 5 is configured as directional magnetic flux antenna 5′ and hasat least 60 dB signal attenuation in directions outside of the specifiedangular region 108.

In one exemplary application of the present invention, system 100 isconfigured to collect event attendee data as shown, for example, inFIG. 1. For example, the system tracks Jean at a trade show orconvention. The data shows that Jean visited booths 31, 32, and 33. Jeanremained at booths 31 and 32 for ninety seconds each; Jean remained atbooth 33 for twenty-eight minutes, twenty-six of those minutes at aparticular display 33 a. Collected data also shows that Jean returned todisplay 33 a at booth 33 an hour later with Chris, the Director of Salesfor Jean's company, and again two hours later with Pat, the companypresident. The operator of booth 33 would then identify a potentialcustomer in Jean's company and would have the information needed tofollow up with Jean for further discussion.

In a variation on the use of system 100, tag readers 1 are placedinconspicuously around the various conference room entrances, therebyallowing data to be collected on the location of objects 2 a holdingtags 2 and the seminars each object 2 a attended. Such a system can beused for tracking and verifying attendance at continuing educationseminars or the like.

Yet another variation, system 100 is used to track guests at a casino.Each casino guest or object 2 a is assigned and given a uniquely codedtag 2. Tag readers 1 are placed inconspicuously at the various gamingareas, thereby allowing data to be collected on the objects 2 a whovisited a particular gaming area and how long each attendee spent in thearea. The collected data can be used by the casino to correlate guestinformation and demographics with gambling habits and use the resultsfor the purpose of improving marketing and operations.

In yet another variation, system is used to track the location of waitstaff at a restaurant for the purpose of measuring customer service andoptimizing diner satisfaction. For example, each table is equipped witha tag reader 1 and each manager and waiter or waitress is an object 2 aequipped with a tag 2. System records how often a manager orwaiter/waitress visits each table, the duration of each visit, and otherdata. Analysis of the wait staff data in combination with otherinformation such as tips, customer turnover frequency, orders received,and diner satisfaction enables the restaurant to adjust wait staffpractices as needed to improve diner satisfaction, evaluate theeffectiveness of promotions, and maximize profits.

Systems and methods of the present invention can be used in many othersituations, including obtaining location data for cattle, employees,packages, luggage, deliveries, and the like.

Turning now to FIG. 17, another aspect of the present invention isdirected to a system 900 for monitoring an environment 902 enclosedpartially or completely by an enclosure 904 made of shielding material.As used herein, the term “shielding material” means metal sheet, wiremesh, or other materials that block or significantly attenuateelectromagnetic waves. In one embodiment, system 900 includes at leastone transmitter 906 disposed in environment 902 that is at leastpartially surrounded by enclosure 904.

In one embodiment, enclosure 904 is made of a material that completelyattenuates (blocks) or attenuates transmission of electromagnetic energyby 80 dB or more (e.g., 92 dB) between a transmitter 906 and a receiver908 to a level where the signal amplitude is on the order of noise orbelow. Such an enclosure can be made of metal sheet or wire mesh. In oneembodiment, environment 902 is enclosed by a wall 905 or walls 905. Oneexample of enclosure 904 is a circular, polygonal, or other closed shapeor substantially-closed shape (e.g., a C or U shape). Wall(s) 905 mayinclude or not include a top and/or bottom.

In many embodiments, enclosure 904 is a three-dimensional rectangularstructure such as a room, building, container, or the like. Any faces ofenclosure 904 not occupied by metal are closed or blocked by the earthto prevent communication using radio waves. In other embodiments,enclosure 904 is a sphere, cylinder, cone, box, or other hollowthree-dimensional shape that is completely closed on all sides byshielding material.

Typically, enclosure 904 is made of metal sheet or wire mesh. Wire mesheffectively blocks transmission of electromagnetic energy when the sizeof openings in the mesh is smaller than the wavelength of theelectromagnetic energy. For system 900, transmitter 906 may be anyembodiment of tag 2 discussed above and receiver 908 may be anyembodiment of tag reader 1 discussed above.

In prior-art temperature-monitoring systems for walk-in refrigerators,for example, a wire passes through the wall of door seal of therefrigerator to a temperature sensor on the inside of the refrigerator.A wired connection is required since radio frequency waves are noteffectively transmitted through the metal-lined walls 905. A sensor nearthe door of a refrigerator often has a wire that passes over the door'ssealing gasket. Over time, the wire and/or the sealing gasket to therefrigerator is weakened by repeatedly opening and closing the door.

Passing a wire through wall 905 of enclosure 904 is problematic to theintegrity of the enclosure and impractical in some situations, such asfor liquid storage tanks. To overcome such problems, system 900 isconfigured to wirelessly transmit temperature, humidity, movement,pressure, flowrate, or any other condition of environment 902 usingnear-field magnetic induction to a receiver on the outside of enclosure904.

Enclosure 904 that is made of metal or lined with metal functions as aFaraday cage to significantly attenuate electromagnetic energy byredirecting these signals to ground 901. For example, environment 902 isthe inside space of a walk-in refrigerator (enclosure 904) with walls905 lined with metal. In the case of RF communication, the metal liningof a refrigerator completely attenuates the RF signal. In contrast, theattenuation of a magnetic field is proportional to distance. Thestrength of a magnetic field is proportional to μ₀/r³, where r is thedistance from the source and μ₀ is the permeability. For air, μ₀=1.0.Copper, iron steel, stainless steel, aluminum, water, concrete, wood andother materials have permeability values that have a negligible effecton magnetic field strength.

Thus, while an RF wave is completely attenuated by enclosure 904 made ofa shielding material, a magnetic field is not. Since the strength of amagnetic field is primarily a function of distance, and to a smallextent a function of the medium through which is passes, the magneticfield of near-field magnetic induction is not attenuated by metal,liquids, or other shielding materials nearly to the extent as occurswith electromagnetic energy such as radio waves. Thus, transmitter 906is capable of effective wireless communication with receiver 908 throughenclosure 904 using near-field magnetic induction.

In one embodiment of system 900, shielding enclosure 904 is arefrigerator or other temperature-controlled enclosure with metal-linedwalls 905. The enclosure's 904 walls 905 (including the ceiling and/orfloor) are typically made of sheet aluminum no thicker than ¼ inch. Insome embodiments, walls 905 include an outer shield 904 a and an innershield 904 b, where inner and outer shields 904 a, 904 b are separatedby foam insulation, air, or other material. In other embodiments,enclosure 904 is a box truck, truck trailer for cargo, a tanker trailer,an oven, an oil tank, a fuel tank, a cargo container, or other structuresurrounding environment 902 on all sides by metal.

A transmitter 906 is coupled to a sensor 907, where transmitter 906 andsensor 907 are disposed in environment 902 within enclosure 904. Areceiver 908 located outside of enclosure 904 is coupled using wire orwirelessly to a network, modem, storage device, computer, display,database, data logger, or other device. For example, receiver 908 iscoupled to a computer for storing temperature readings and to a displayfor communicating the condition of environment 902 to viewers.Transmitter 906 and receiver 908 are configured to communicate usingnear-field magnetic induction. In one embodiment, transmitter 906 andreceiver 908 communicate using near-field magnetic induction with afrequency of 100-300 KHz. In some embodiments, transmitter 906 andreceiver 908 are separated by the short distance through wall 905 of theshielded enclosure 904.

One example of system 900 in use is a walk-in freezer or cooler. Sensor907 is a thermocouple connected to transmitter 906 located inenvironment 902 inside the freezer. Receiver 908 communicates withtransmitter 906 via near-field magnetic induction. Receiver 908 iscoupled to a computer 911 and to a display 913 for temperature. Computer911 is programmed to communicate a message or alarm if the temperatureof environment exceeds or falls below a set temperature or acceptabletemperature range. Workers on the outside of the freezer can also seethe temperature on the display 913, which may be color-coded. Forexample, temperatures within the user-set acceptable temperature rangeare displayed in green while temperatures outside the acceptabletemperature range are displayed in red. The display may also flash orsound an audible alarm when the temperature is outside of the acceptabletemperature range.

Although the preferred embodiments of the present invention have beendescribed herein, the above description is merely illustrative. Furthermodification of the invention herein disclosed will occur to thoseskilled in the respective arts and all such modifications are deemed tobe within the scope of the invention as defined by the appended claims.

The invention claimed is:
 1. A system for collecting data about objectsmoving within a specified site: at least one tag reader disposed withina specified site and configured to transmit a magnetic field to aninterrogation area within the specified location and extending at leasttwo meters across an open-air gap, the at least one tag reader having atransceiver antenna comprising: a first antenna coil disposed on aplanar substrate and substantially defining a first coil closed geometrywith an innermost first coil turn and an outermost first coil turn; atleast one second antenna coil disposed on the planar substrate withinthe innermost first coil turn and having a plurality of second coilturns each defining a substantially closed second coil geometry, whereinthe at least one second antenna coil defines an effective second coilarea; and at least one third antenna coil disposed on the planarsubstrate outside of the outermost first coil turn and having aplurality of third coil turns defining a substantially closed third coilgeometry, wherein the at least one third antenna coil has an effectivethird antenna coil area; wherein the at least one third antenna coil isconnected in series with the at least one second antenna coil andwherein the effective third antenna coil area differs from the effectivesecond coil area by no more than 5%, thereby resulting in at least 95%phase cancellation between a second voltage induced in the at least onesecond antenna coil and a third voltage induced in the at least onethird antenna coil when conducting a current through the first antennacoil; and a plurality of tags configured to wirelessly communicate withthe at least one tag reader via near-field magnetic induction across anopen air gap of at least 2 meters, wherein each of the plurality of tagsis constructed to be carried by an object moving within the specifiedsite, wherein when located within the interrogation area, each of theplurality of tags is configured to wirelessly communicate a uniqueidentifier to the at least one tag reader via near-field magneticinduction.
 2. The system of claim 1, wherein the first antenna coil is atransmitter antenna and wherein the at least one second antenna coil andthe at least one third antenna coil comprise a receiver antenna.
 3. Thesystem of claim 1, wherein the first antenna coil is a receiver antennaand wherein the at least one second antenna coil and the at least onethird antenna coil comprise a transmitter antenna.
 4. The system ofclaim 1, wherein the effective third antenna coil area differs from theeffective second coil area by no more than 1%, thereby resulting in atleast 99% phase cancellation between the second voltage induced in theat least one second antenna coil and the third voltage induced in the atleast one third antenna coil when conducting the current through thefirst antenna coil.
 5. The system of claim 1, wherein the transceiverantenna is configured as a focused-beam directional antenna constructedand arranged to direct the magnetic field within a particular angularregion to define the interrogation area.
 6. The system of claim 1,wherein the magnetic induction has a frequency from 100 KHz to 300 KHz.7. The system of claim 1, wherein the at least one tag reader isconfigured to transmit and receive at the same time.
 8. A method ofcollecting location data from objects moving within a specified sitecomprising: providing at least one tag reader with a tag reader antennaconfigured to receive signals via near-field magnetic induction acrossan open-air gap of at least two meters; providing a plurality of tagsconfigured to wirelessly communicate with the at least one tag readervia near-field magnetic induction across an open air gap of at least 2meters; the at least one tag reader listening for the tag signaltransmitted by any one or more of the plurality of tags; each of theplurality of tags transmitting a tag signal containing a uniqueidentifier; receiving a tag signal from a particular tag at the at leastone tag reader; demodulating the tag signal from the particular tag todetermine the unique identifier associated with the particular tag; andsaving the unique identifier.
 9. The method of claim 8, wherein theproviding step includes selecting the tag reader with the tag readerantenna configured as a transceiver antenna capable of transmitting andreceiving at the same time.
 10. The method of claim 8, wherein the tagsignal comprises a plurality of tag signals separated in time by aconstant tag signal interval.
 11. The method of claim 8, wherein the tagsignal comprises a plurality of tag signals separated in time by avarying tag signal interval.
 12. The method of claim 8, wherein the tagsignal comprises a plurality of tag signals separated in time by a tagsignal interval that is adjustable by a user.
 13. The method of claim 8,wherein the magnetic induction has a frequency from 100 KHz to 300 KHz.14. The method of claim 8, further comprising: selecting the at leastone tag reader configured to transmit a magnetic field to aninterrogation area extending across an open-air gap of at least twometers; the at least one tag reader transmitting the magnetic field tothe interrogation area; and at least one of the plurality of tagsreceiving the magnetic field.
 15. The method of claim 14, wherein thetag reader antenna is configured as a directional antenna and theinterrogation area is substantially constrained to a specific angularregion.
 16. The method of claim 8 further comprising equipping at leastone object with one of the plurality of tags.
 17. A method of monitoringan enclosed environment comprising: disposing a sensor inside anenvironment enclosed by an enclosure made of metal; disposing atransmitter inside the environment, the transmitter configured tocommunicate using near-field magnetic induction; coupling the sensor tothe transmitter; disposing a receiver outside the enclosure, thereceiver configured to communicate with the transmitter using near-fieldmagnetic induction; coupling the receiver to a communications device;wirelessly communicating a sensed condition of the environment from thetransmitter to the receiver using near-field magnetic induction; andcommunicating the sensed condition to a user using the communicationsdevice.
 18. The method of claim 17, wherein the communications device isselected from the group consisting of a computer, a display panel, awireless network, and a cabled network.
 19. The method of claim 17,wherein the enclosure is selected from the group consisting of arefrigerator, a freezer, an oven, a liquid-storage tank, a cargotrailer, and a cargo container.